1. Field of Invention
Embodiments of the present invention relate to semiconductor memory devices and methods of fabricating the same, and more particularly, to a semiconductor device such as a phase change memory device having a diode and a method of fabricating the same.
2. Description of the Related Art
A phase change memory device is a type of memory device using resistance difference caused by phase change within a phase change material, and is a nonvolatile memory device. One unitary cell of such a phase change memory device includes a switching device and a phase change resistor electrically connected to the switching device.
The switching device may be a metal-oxide semiconductor (MOS) transistor. If the switching device is a MOS transistor, a large program current of at least several mA is necessary to program a unitary cell of the phase change memory device. Moreover, the program current must flow through the MOS transistor. Therefore, there is a limit in reducing the area occupied by the MOS transistor. Accordingly, if a MOS transistor is used as the switching device, there may be a limit in improving integration of the phase change memory device.
Instead of the MOS transistor, a vertical PN diode may be used as the switching device of a unitary cell to solve the aforementioned problems. However, since the threshold voltage of the vertical PN diode is rather high (between 0.7V and 0.8V), it is difficult to reduce the operating voltage of the vertical PN diode. Also, since the program current is carried by holes in the vertical PN diode, the on-current of the PN diode is low. As a result, a programming efficiency is relatively poor when the vertical PN diode is used as the switching device.